1. Field of the Invention
The present invention relates to a sheet conveying apparatus and an image forming apparatus having the same, in particular, to a material and shape of a pinch roller which is provided in the sheet conveying apparatus and constitutes a component of a pair of conveying rollers for pinching and conveying a sheet.
2. Related Background Art
In the past, an image forming apparatus such as printer, copying machine and facsimile has been provided with a sheet conveying apparatus for conveying a sheet to an image forming portion, and as the sheet conveying apparatus, known is the type which is provided with a pair of conveying rollers comprising a rubber driving roller which comes in contact with a back surface of the sheet and a pinch roller which presses the sheet on the rubber driving roller.
In regard to the rubber roller, the limits of deflection and outside diameter accuracy are low and outside-diameter changes with temperature change are large. In order to cope with this situation, as the configuration for obtaining much higher conveying accuracy, there is provided a sheet conveying apparatus configured by using, as the driving roller, a metal roller having roughness provided on the surface thereof and, as the driven pinch roller, a rubber roller having a fluorine coating applied on the surface thereof as disclosed in, for example, Japanese Patent Application Laid-Open No. 11-013744.
At this point, with the roughness provided on the metal roller surface, the metal roller can convey a sheet because the roughness bites into the back surface of the sheet. And the fluorine coating applied on the rubber roller surface not only enables the prevention of the plasticizer contained in the rubber from being eluted and deposited on the sheet surface, but also enables the decrease in xcexc value (frictional resistance value) of the rubber. Thus the leading end of the sheet is allowed to enter the nip portion between the conveying roller and the pinch roller without failure.
At the time of using a rubber roller, as the pinch roller, which comes in pressure contact with a conveying roller rotating in the opposite direction to the conveying direction of a sheet so as to correct skew feed of the sheet, the leading end of the sheet is allowed to be prevented from folding when it strikes the pinch roller during the skew feed correcting operation by decreasing xcexc value of the rubber roller surface in such a manner as to apply a fluorine coating on the surface of the rubber roller.
The sheet conveying apparatuses of the conventional art which are provided with the pinch roller as described above, however, give rise to problems that applying a fluorine coating on the rubber roller surface is very costly and that the fluorine coating wears easily, accordingly, the durability of the pinch roller is low.
Next, the state in which the fluorine coating wears will be briefly described with reference to FIG. 7. In FIG. 7, reference numeral 8 denotes a conveying roller having a roughness 8a provided on its surface and numeral 16 a pinch roller driven to rotate by coming in pressure contact with the conveying roller 8, and the pinch roller 16 includes a rubber roller 16a and a fluorine coating layer 16b formed on the rubber roller 16a. 
Furthermore, the conveying roller 8 and the pinch roller 16 are designed to rotate in the opposite direction to the conveying direction of a sheet, as indicated by an arrow B, at the time of starting the sheet feeding operation. And correction of skew feeding is performed in such a manner as to form a loop of a sheet S1 by allowing the leading end of the sheet S1 conveyed by a sheet feeding mechanism not shown in the figure to strike the nip portion between the conveying roller 8 and the pinch roller 16 both of which are rotating.
During this correction of skew feeding, since the pinch roller 16 rotates against the leading edge portion of the sheet S1, the fluorine coating layer 16b applied on the surface of the pinch roller 16 is abraded by the leading edge portion of the sheet S1 and wears gradually.
Although the state in which the fluorine coating wears has been described in terms of the correction of skew feed in reverse rotation in which the sheet S1 is allowed to strike the conveying roller 8 and the pinch roller 16 both rotating in the reverse direction, the fluorine coating layer 16b also wears during the correction of skew feed in stopping rotation in which the sheet S1 is allowed to strike the conveying roller 8 and the pinch roller 16 both in a stopped state, though the degree of wearing differs from each other.
When the fluorine coating layer 16b becomes worn as described above, the rubber roller 16a becomes exposed with the increase in the number of sheets passing through the apparatus, which causes deposition of the plasticizer on the sheet S1 and folding of the leading end of the same.
The present invention has been achieved in light of the situation as described above; accordingly, the object of the present invention is to provide a sheet conveying apparatus of low cost and high durability and an image forming apparatus having the same.
The present invention provides a sheet conveying apparatus including a pair of conveying rollers for pinching and conveying a sheet by means of a metal driving roller having roughness provided on a surface thereof and a pinch roller coming in pressure contact with and being driven by the metal driving roller to be rotated, in which the pinch roller is formed of a synthetic resin or a resin.
In the sheet conveying apparatus, the pinch roller is formed in such a manner that the outside diameter of the opposed end portions of the pinch roller is smaller than that of the other portion of the pinch roller.
In the sheet conveying apparatus, the pinch roller is formed in such a manner that the outside diameter of the opposed end portions of the pinch roller is smaller than that of the substantial center portion of the pinch roller and that the opposed end portions and the center portion have a smoothly connected shape.
In the sheet conveying apparatus, the outside diameter of the opposed end portions of the pinch roller is smaller than that of the substantial center portion by 10 xcexcm or more.
In the sheet conveying apparatus, the opposed end portions of the pinch roller have a round-shape.
In the sheet conveying apparatus, the pinch roller is formed of the material of which main raw material is polyacetal.
In the sheet conveying apparatus, the pinch roller is formed with a die, the die is parted in the longitudinal direction so that the parting line will not appear on the circumference of the pinch roller, the inside diameter of the die part for forming the end portion near the core side of the pinch roller is smaller than that of the portion for forming the substantial center portion of the pinch roller, and the inside diameter of the die is set in such a manner that galling is not caused between the pinch roller and the die during the mold releasing in expectation of some percentage for shrinkage of the substantial center portion of the pinch roller during the resin fillingxe2x80x94coolingxe2x80x94releasing processes.
In the sheet conveying apparatus, the roughness is provided on the surface of the driving roller by coating the surface of a metal roller with ceramic powder or alumina powder.
In the sheet conveying apparatus, the roughness is provided on the surface of the driving roller by subjecting the surface of a metal roller to blast finishing.
In the sheet conveying apparatus, the roughness is provided on the surface of the driving roller by subjecting the surface of a metal roller to knurling.
Further, the present invention is an image forming apparatus including an image forming portion and a sheet conveying apparatus for conveying a sheet to the image forming portion, and as the sheet conveying apparatus, any one of the apparatus described above is used.
And, the sheet is pinched and conveyed by a metal driving roller having roughness provided on its surface and a pinch roller coming in pressure contact with the driving roller and thereby driven to rotate. The pinch roller is formed of a synthetic resin or a resin, thereby the pinch roller can be manufactured less costly and the endurance of the same can be increased.